Absorption of carbon dioxide from gases containing the same



1953 F. PORTER ETAL ABSORPTION OF CARBON DIOXIDE FROM GASES CONTAININGTHE SAME Filed May 2, 1950 S K C \m m E @EDQ N R D E E R .2533 W T O 558 I WW P U K C mwuz tuxu bit Q23 N N 5303 A H R O NQU F J 635523 Y Bmmmmomm dang ZTTORNEY.

Patented Aug. 18, 1953 UNITED S'i EELS FA'E'EN'E' OFFICE ABSORPTION OFCARBON DIOXIDE FROM GASES CONTAINING THE SAME Application May 2, 1950,Serial No. 159,529

3 Claims.

This invention relates to the absorption of carbon dioxide in liquidsolvents and more particularly refers to a new and improved method ofremoving carbon dioxide from gaseous mixtures containing the same.

In the preparation of gases for synthesis of ammonia whether by partialcombustion of hydrocarbons, water gas reaction or the like there isproduced a gas mixture containing a high percentage of carbon dioxidewhich must be removed since gases having an appreciable amount of carbondioxide cannot be tolerated in the ammonia synthesis. Alkaline solutionsmay be employed as a reactant to extract carbon dioxide from gasmixtures; however, thepreferred commercial method of treatinghydrogen-carbon dioxide mixtures is by absorption of the carbon dioxidein water. The relatively low absorption coefficient of water for carbondioxide (0.88 volume CO2 absorbed at 1 atmosphere at,20 C. in 1 volumewater) imposes the requirement of a high ratio of water to gas to effectremoval of carbon dioxide with of course large equipment and operatingcosts. Mere substitution of any solvent having a higher absorptioncoefiicient than water does not suffice since there are many otherfactors which bear on the suitability of a solvent for absorption ofcarbon dioxide as for example resistance to oxidation by air, resistanceto polymerization, inertness to sulfur compounds, heat of solution,absorption coefficient for hydrogen, vapor pressure, freezing point, andviscosity. Further, a solvent may have physical properties deemeddesirable for scrubbing carbon dioxide containing gases but in practicalapplication prove to be a failure under the operating conditions imposedthereby. Thus, it will be evident that successful commercial extractionof carbon dioxide from gases involves the use of a solvent havingdesired physical properties and particularly a method of operation whichwill utilize such solvent in an efiicient and economical manner.

An object of the present invention is to provide a simple and efficientmethod for scrubbing carbon dioxide from gases containing the same witha normally liquid, nonreactive organic solvent.

method of absorbing carbon dioxide in a normally liquid, non-reactiveorganic solvent having a higher absorption coefiicient for carbondioxide than water and concomitantly maintaining the absorptioncoefiicient of the solvent near its maximum value.

A further object of the present invention is to provide a normallyliquid, non-reactive oranic solvent having relatively high absorptioncoeflicient for carbon dioxide as compared to water together with otherproperties which make it eminently suitable for scrubbing carbon dioxidefrom gas mixtures.

Other objects and advantages will be apparent from the followingdescription and accompanying drawing.

A satisfactory solvent for the absorption of carbon dioxide shouldpreferably have the following properties:

(a) a fairly high absorption coefficient for carbon dioxide, preferablyabout four times that of watertoo low an absorption coeflicient wouldnot warrant its cost;

(1)) no salt formation with the carbon dioxide, since then a greatercost than mere degasification would be encountered in recovering thesolvent for reuse;

.(c) a low absorption coefficient for hydrogen so as not to lose anappreciable quantity of hyclrogen;

(d) an inertness to reaction with sulfur compounds so thatdegasification could be used to recover the solvent;

(6) a low absorption coefficient for oxygen so as not to absorb oxygenduring the degasification by means of an air stream and then releaseoxygen'into the product gas during the scrubbing step;

(f) a resistance to oxidation by air so as not to destroy the solventduring the degasification step;

(9) a 10W vapor pressure in order to secure a minimum loss duringabsorption and degasification;

(h) a freezing point at or below 0 C. so as not to freeze in coldweather;

(k) a heat of solution for carbon dioxide not appreciably higher thanwith water.

(CEO CHzCHzO] 4CH3) since it has outstanding propertiesasjcompared",

with water and in addition is available in commercial quantities at arelativelyzlowacost. The; table below gives acomparisoniofftheipropentiesx of water and dimethoxytetraethylen'eglycol:

. eth' or Physical Properties Water g g g g Density, D20 0. 998 1.013Boiling Point in C. @760 mm Y 100- 276; Vapor Pressure in mm. Hg 20 C.17. 53 0.0921 Viscosity in Ops. @20 0.. 1.00 3. 93- CorAbsl Goeil; @20O. 0. 88 4.00;- Hz Abs, Coefh 20 C. O. 021 O. 039 Heat of-solution 2 4,755 4, 920

1 Absorption coefficient for gas insolvent at 20 0.; volume (N: T.

P.) of gas-absorbed at one atmosphere (total pressure of'gas'plus'solvent) per volume of solvent:

1 Heat of solution in gram calories for gram mole of carbon dioxideFromthetable above'it will be'apparent that dimethoxytetraethyleneglycol is" far superiorto.

water with respect to its vapor pressureand its absorption coefficientfor carbon" dioxide: Although the hydrogen absorption" coefficient ofdimethoxytetraethylene glycol is slightly greater than-thecomparable'figure forwater; in -practice' le'ss' hydrogen (the desiredproduct: of: the process) will be lost when usingGiIIIGUI'IOXYilEtIEiiethyleneglycol due to its higher CO2 absorptionco'efiicient' because less liquid solvent isarequired" to remove thesamequantity of" carbon dioxide from the gases. The minor diiTerence-ircheatof'sol'ution between dimethoxytetraethylene glycol and water will have anegligibleeffect on the operation.

In practical tests employing ethers'of'pol'yglycols as solvents forscrubbing carbon dioxide from gases we found the CO2 absorptionc'oeflioient of the solvent dropped rapidly and to such an' extent so asto make the process commercially impractical. After extensiveinvestigation we" discovered that moisture was absorbe'd by the solventfrom thegases undergoing treatment and from the air blast employed todegasifythe solventl Smallamountsof water exerted an unexpected"retarding effect upon the absorption coefficient of the organic solvent.For example;- normally one would expect that a blendof 90%dimethox-yt'etraethylene glycol with water would have an absorptioncoefficient in propor" tion' to the; absorption coefficient exerted bythe individual components, i. e. 90% of 410:35; and 10% of 0.88:0.09. ora combined eiiect. of 3169'. By measurement the absorption. coefiicient,of the. blendsurprisi-ngly was found to be 2.35. Dimethoxytetraethyleneglycol diluted with.5.%. water has an absorption coeihcient of3.44andwhen diluted with 2- water has-an absorptioncQ- efiicient of3.78. Our testshave demonstrated.

4,. the importance and indeed the necessity in commercial operation ofmaintaining the water content of the liquid organic solvent to as low anamount as practical, preferably below 5%.

In accordance with the present invention removal of carbon dioxide fromgases containing the same may be accomplished by passing a gascontaining carbon dioxide in intimate contact with a normally liquidpolyglycol ether thereby absorbing the carbon dioxide in the polyglycolether. solvent, degasifying. thersolvent bypassing air through" thesolvent-j to. remove the carbon dioxide dissolved in the solvent,removing water from the solvent in an amount such that the resultingsolvent contains less than 5% water, and returning the: tlius= degassed,dewatered solvent for further contact with the gas containingcarbondioxide.

A specificembodiment of the invention comprises. passing ahydrogen-carbon dioxide gas mixtureacountercurrent and in intimatecontact with dimethoxytetraethylene glycol in a first zone; withdrawingthe dimethoxytetraethylene glyoolgafter contact with the gas from thefirst zone and passing the dimeth-oxytetraethylene glycol. containingdissolvedcarbon dioxide into arsecondzonein contact witha stream of airto efi'ectLremovalofthe. carbondioxide from the dimethoxytetraethyleneglycol, withdrawing a portion of the degasified dimethoxytetraethyleneglycolfromthe secondzone and-evaporating water from saidpor-tionin a.third zone, comminglingthe. dewatered. dimethoxytetraethylen glycoliromthe third .zone wtih the remaining por' tion. of degasifieddimethoxytetraethylene glycol fromthe secondzone and.returning-themixture tothe firstzone-forfurthercontact with the hydrogen carbon.dioxide gas mixture, and correlating, the relative, proportion ofdegasified dimethoxytetraethylene glycol. withdrawn from the second.zone for waterremoval with the amountoiwater. removedtherefrom in thethird zone so that themixtureof; degasified dime-thoxytetraethyleneglycol from the second zone not subjected: towaterremoval with dewateredclimethoxytetraethylene'glycol'after water removal from the thirdzonewill contain less than 2 waten The'accompanying draw-ingis adiagrammatic flow v sheet. illustrating one 1 method of practicingthepresent. invention.

Referring. to-thedrawing, hydrogen-carbon dioxide-gasmixtures at leastpartially saturated with water. vaporand containing small amounts of,other impurities such as J hydrogen.- sulfide and car-hon: monoxideare,- introduced at a low tem--' perature andsuperatmosphericpressurethrough line I" into the bottom of-= absorptiontower- 2: Althougha. conventional absorption tower in which-liquid issprayed into the top ofthe tower and gas introducednear the bottom ofthe tower passes upwardly countercurrent to: the liquidisillustrated-,-v any suitable deviceior obtaining intimatercontact between the gas andthe liquidmay be. employed. More efficient scrubbing of the gases may be-obtained by utilizing aplurality. of absorption columns connected:series. Superatmospheric pressure is desirablymaintained in column -2since pressure hasthe effect of increasingthe-amount ofcarbondioxide-absorbed by the solvent andincreasing the rate atwhichthecarbondioxideis absorbed. Solubility of carbon dioxide in the solventdecreases with. increased temperature. While subatmospherictemperatureswillincreasethe. solubility: of. carbon diox.--

ide in the solvent the maintenance of such low temperatures wouldrequire the use of refrigeration which would add considerably to thecost of operation. A further disadvantage in maintaining subatmospherictemperatures would be the increase in viscosity of the solvent. For goodcommercial practice we have found room temperature C.) in the towersatisfactory.

Solvent at a temperature preferably below 20 C., is introduced into thetop of CO2 absorber 2 through line 3 and sprayed downwardlycountercurrent to the upwardly rising gas mixture containing CO2.Scrubbed gas is released from the top of the tower 2 through line 4. Asa result of the heat solution of carbon dioxide in the solvent thetemperature of the solvent is elevated several degrees.

Liquid solvent containing dissolved carbon dioxide at a temperatureseveral degrees above 20 C. is withdrawn from absorber 2 through line 5,cooled to about 20 C. in conventional cooler 6 and reduced in pressureto approximately atmospheric pressure by passing through reducing valveI. For the sake of economy the energy available due to the reduction inpressure through valve I may be utilized, for example to drive a Peltonwheel, which latter may be connected to a dynamo to generateelectricity. Also, as a result of reduction in pressure the temperatureof the solvent passing through valve 1 drops about 5 to 10 C.

The pressure reduced solvent flows through line 9 into the top ofdesorber I I, which may be any conventional column such as bubble-captype tower or packed tower. More rapid and complete degasificationoccurs at lower pressures. To avoid the use of vacuum pumps we prefer tomaintain substantially atmospheric pressure on tower I I. A pool ofliquid is desirably allowed to accumulate in the bottom of desorber IIfor the purpose of maintaining a liquid seal therein and to act in thenature of a surge tank to compensate for fluctuations in rate ofcirculation of liquid solvent. Atmospheric air which of course containsmoisture is forced through line I2 into the bottom of column II passingupwardly countercurrent to the how of solvent entering through line 9.This operation is referred to as degasification because the air streamremoves carbon dioxide from the solvent in column I I and carries thecarbon dioxide out of the system through line I3. In general an amountof air equal to twice the volume of carbon dioxide dissolved in thesolvent should be sufficient to remove substantially all the carbondioxide in the solvent.

The use of an air stream for accomplishing degasification of the solventover other methods, as for example heating the solvent to drive off thecarbon dioxide, has the advantages of simplicity and economy whichincludes savings in fuel to heat the solvent and cooling water to reducethe heated solvent to room temperature before entry in the absorptioncolumn. Also, avoidance of high temperature minimizes any possibility ofdeterioration of solvent.

A portion of the degasified solvent withdrawn from the bottom of columnII through line I4 is forced by pump I5 through line I6, heat exchangerI'I into evaporator I8. In heat exchanger H the hot liquid productleaving evaporator it passes in indirect contact with degasified solventthereby cooling the evaporator bottoms and heating the degasifiedsolvent thus effecting a heat economy. The function of evaporator I8 isto vaporize water from the degasified solvent entering therein. Aconventional form of evaporator as illustrated in the drawing consistsof a shell I9 with one or several bubble-cap type trays 2I and a steamcoil or jacket 22 into which steam enters through line 23 and watercondensate discharges through line 24. To facilitate removal of waterand minimize decomposition and evaporation of the solvent, it isdesirable to operate evaporator I8 under subatmospheric pressure. Ajettype vacuum pump 25 may be employed for this purpose with steamentering line 26 as the propelling medium for inducing suction andcarrying water vapor from evaporator I8 through line 21. When a vacuumis imposed on evaporator I8 a low temperature of about C. at the bottomof vessel I8 will ordinarily be suflicient to drive moisture from thesolvent. Another advantage of maintaining low temperature in evaporatorI8 is that low pressure steam, usually a by-product in the plant, willbe adequate to vaporize the water from the solvent.

Solvent after removal of water therefrom is withdrawn from the bottom ofevaporator I8 through line 28, passing through heat exchanger I! whereinit gives up a large part of its heat to the incoming solvent, and thencethrough line 29 into cooler 3I wherein the solvent is further cooled toabout room temperature (20 C.) The cooled solvent flows from cooler 3Ithrough line 32 where it commingles with degasified solvent leaving CO2desorber II through lines I4 and 33, and the mixture flowing throughline 34 is returned by pump 35, line 3 to the top of CO2 absorber forfurther scrubbing of the incoming gases.

For best operation the solvent entering the top of absorber 2 shouldcontain no more than 5% water. As a practical matter we have found itdesirable to retain a small amount of water, about 2 70, in thecirculating body of solvent. In general it is necessary to bleed fromCO2 desorber II only a small portion of the liquid solvent, i. e. lessthan 4%, and usually 1 to 2%, of the circulating body of solvent andsubject such small portion of liquid solvent to evaporation to remove anamount of water which will balance the water vapor carried into thesystem by the incoming ases and air and thereby maintain the system inequilibrium with respect to the water content in the circulating body ofsolvent. Of course the entire body of solvent could be heated to stripit of water but this would entail a large increase in operating andequipment costs and further would subject a larger body of the solventto higher temperatures with increased solvent losses and possibledegradation of solvent. The advantages of bleeding only a relativelysmall portion of solvent from the system and subjecting it toevaporation are thus apparent.

As previously stated hydrogen-carbon dioxide mixtures commerciallyemployed in the prepa ration of ammonia synthesis gases contain smallamounts of sulfur impurities particularly hydrogen sulfide. Since suchsulfur compounds have been known to readily react with organic compoundsit was necessary in order to determine the suitability of the others ofpolyglycols as a solvent to ascertain what effect the sulfur compoundswould have on the polyglycol ethers. Ethers of polyglycols, specificallydimethoxytetraethylene glycol, were admixed with sulfur compounds andmaintained at 100 C. for long periods of time. It was found that theethers of polyglycols, show. no significant deterioration: as

a result: of: such treatment.

@th'entests: were conducted; with. the others of polyglycols. todetermine: whetherthey would oxidize; polymerize, or decompose underthecon-- diiio'ns of: operation. The polyglycol ethersowere Hydrogen-carbondioxide gas mixture Hydrogen 49.45 Carbon dioxi 29.00 Carbon monoxide5.00 Hydrogen sulfide 0.05 Nitrogen, argon, etc 16.50

Total 100.00 Water in gas mixture ounces per 1000 cu. ft 0.64

Dimethoxytetraethylene glycol containing 2 by weight water at atemperature of 13 C. is pumped into the top of the absorption towermaintained at a pressure of 435 pounds per square inch. gauge at therate or" 18,000 gallons solvent per hour. Scrubbed gases are releasedfrom the top of the absorption tower. Liquid solvent containingdissolved carbon dioxide at a temperature of about 22 C. by virtue ofthe heat of solution of carbon dioxide in the solvent is withdrawn fromthe bottom of the absorption column, cooled to 20 C. and then reduced inpressure to approximately pounds per square inch gauge. As a result ofthe reduction in pressure of the liquid solvent the temperature of themixture of dimethoxytetraethylene glycol and dissolved carbon dioxide isfurther reduced to about 125 C. The low pressure solvent and'carbondioxide mixture passes into the top of a desorber column containingbubble-cap trays maintained at 15 pounds per square inch gauge. A streamof atmospheric air containing 1 pound 9 ounces water per 1000 cu. ft. isblown at the rate of 22,500 cu. ft. per hour measured at standardconditions upwardly through the desorber countercurrent to the how ofsolvent containing carbon dioxide. Air and carbon dioxide are'releasedfrom the top of the desorber column. 270 gallons per hour of degasifiedsolvent from the bottom of the desorber column are withdrawn, preheatedto a temperature of 75 and introduced into an evaporator maintainedunder a vacuum or" 60 mm. Hg and at a temperature of 103 C. by means oflow pressure steam. Approximately 36 pounds of water per hour, an amountabout equal to water added to the system by virtue of the introductionof moisture contained in the gas undergoing treatment and the airen'iployed for degasification, are removed from the top of theevaporator. Ihe dewatered solvent from the evaporator is cooled to aboutC. and commingled with the remaining portion of degasified solvent fromthe bottom of. the desorber and the mixture then returned to, the top ofthe absorption column for further scrubbing.

During the initial stage of the operation the solvent: is permitted to.accumulate 2 by."

weight of water and thereafter sufii'cient' wateris removed fromthe-evaporator to maintain this:

changes andmodifications may be made therein without departing from thescope and spirit of the invention;

We claim:

1. A process for the removal ofcarbon-dioxide from gases containing thesame which comprises passing a gas containing carbon dioxide in intimatecontact with a normally liquid polyglycol ether selected from the groupconsisting of di-- methoxytetraethylene glycol, diethoxytriethyleneglycol, dibutoxytriethylene glycol, dibutoxydiethylene glycol,dipropoxytetraethylene glycol, dipropoxytriethylene glycol anddipropoxydiethylene glycol thereby absorbing the carbon dioxide in thepolyglycolethersolvent, passing a stream of air through the polyglycolether containing dissolved carbon dioxide to effect removal of thecarbon dioxide from the solvent, subjecting a minor portion of thedegassed solvent to evaporation for removal of Water therefrom,commingling the dewatered solvent with degassed solvent not subjected toevaporation for removal of water, returning the mixture or. devvateredand degassed solvent for further contact with the gas containing carbondioxide, and regulating the, relative proportion of. degassed solventand dewatered solvent in the mixture so that the solvent mixture willcontain less than 5% b volume of water.

2. A process for the removal of carbon dioxide from gases containingthesame which comprises passing a gas containing. carbon dioxide inintimate contact with dimethoxytetraethylene glycol thereby absorbingthe carbon dioxide in the dimethoxytetraethylene glycol solvent, passinga stream of air through.the,dimethoxytetraethylene glycol containingdissolved carbon dioxide to effect removal of the carbon dioxide fromthe solvent, subjecting a minor portion of the degassed solvent toevaporation for removal of. Water. therefrom, commingling the dewateredsolvent with degassed solvent not subjected to evaporation for removalof Water, returning the mixture of dewatered and degassed solvent forfurther contact with the gas containing carbon dioxide, and regulatingthe relative proportion of degassed solvent and dewatered solvent in themixture so that the solvent mixture will contain less than 5% by volumeof water.

3. A process for the removal of carbon dioxide from gases containing thesame which comprises passing a gas containing carbon dioxide in intimatecontact with diethoxytriethylene glycol thereby absorbing the carbondioxide in the diethoxytriethylene glycol solvent, passing a stream ofair through the diethoxytriethylene glycol containing dissolved carbondioxide to efiect re moval of the carbon dioxide from the solvent,subjecting a, minor portion of the degassed solvent to evaporation-for.removal of water therefrom, commingling the dewatered solvent withdegassed solvent not subjected to evaporation for removal. of water,returning the mixture of dewatered and degassed. solvent for furthercontact'with the gas containing carbon dioxide, and regulatingtherel'ativeproportion of degassed solvent: and dewatered 'solvent inthe mixture so that 9 the solvent mixture will contain less than 5% byvolume of water.

FRANK PORTER. JOHN C. ECK.

References Cited in the file of this patent UNITED STATES PATENTS NameDate Millar et a1 July 13, 1937 10 Millar et a1. Dec. 6, 1938 NumberNumber Number

1. A PROCESS FOR THE REMOVAL OF CARBON DIOXIDE FROM GASES CONTAINING THESAME WHICH COMPRISES PASSING A GAS CONTAINING CARBON DIOXIDE IN INTIMATECONTACT WITH A NORMALLY LIQUID POLYGLYCOL ETHER SELECTED FROM THE GROUPCONSISTING OF DIMETHOXYTETRAETHYLENE GLYCOL, DIETHOXYTRIETHYLENE GLYCOL,DIBUTOXYTRIETHYLENE GLYCOL, DIBUTOXYDIETHYLENE GLYCOL,DIPROPOXYTETRAETHYLENE GLYCOL, DIPROPOXYTRIETHYLENE GLYCOL ANDDIPROPXYDIETHYLENE GLYCOL THEREBY ABSORBING THE CARBON DIOXIDE IN THEPOLYGLYCOL ETHER SOLVENT, PASSING A STREAM OF AIR THROUGH THE POLYGLYCOLETHER CONTAINING DISSOLVED CARBON DIOXIDE TO EFFECT REMOVAL OF THECARBON DIOXIDE FROM THE SOLVENT, SUBJECTING A MINOR PORTION OF THEDEGASSED SOLVENT TO EVAPORATION FOR REMOVAL OF WATER THEREFROM,COMMINGLING THE DEWATERED SOLVENT WITH DEGASSED SOLVENT NOT SUBJECTINGTO EVAPORATION FOR REMOVAL OF WATER RETURNING THE MIXTURE OF DEWATEREDAND DEGASSED SOLVENT FOR FURTHER CONTACT WITH THE GAS CONTAINING CARBONDIOXIDE, AND REGULATING THE RELATIVE PROPORTION OF DEGASSED SOLVENT ANDDEWATERED SOLVENT IN THE MIXTURE SO THAT THE SOLVENT MIXTURE WILLCONTAIN LESS THAN 5% BY VOLUME OF WATER.